Crohn's disease is a lifelong condition resulting in inflammation of the bowel. Treatment with powerful drugs aim to reduce inflammation by suppressing the immune system. It is important to regularly assess if the drugs are effective, so they can be stopped or changed if not.

There are several ways doctors assess if the medication is effective as just relying on how the patient feels is not sufficient. Blood and stool tests looking for inflammation are useful but have limitations. Looking into the bowel using endoscopy is effective but very invasive. Medical imaging notably MR Enterography (MRE), a type of MRI scan, and intestinal ultrasound (IUS) visualise the bowel, and are safe, non-invasive and generally well tolerated tests frequently used in the NHS to assess how well treatment is working.

There is however wide variation in which imaging test is selected without clear guidance. The investigators currently do not know if one is better in assessing response to treatment, and if they can be used interchangeably.

The proposed study will investigate this question to guide the future care of Crohn's disease patients. The investigators will recruit patients having follow up imaging tests to assess their treatment and ask them to undergo both MRE and IUS. Radiologists will interpret the MRE and IUS, unaware of the findings of the other test, and state if the disease is improving or not. The doctors looking after the patient will then look at all available information e.g. symptoms, blood/stool tests, colonoscopy, and decide if the patient is the same, worse or better on the medication. This decision will be used to establish how accurate MRE and IUS are in assessing response, how often they agree and if they can be used interchangeably in the future, for example based on patient preference or availability at the hospital.

Imaging, typically with MR enterography (MRE) and/or Intestinal Ultrasound (IUS), is central in the follow up of Crohn's disease patients to assess treatment response to powerful immunosuppressive therapy. Practice varies widely on which imaging modality is selected and/or available and if they are used in isolation or interchangeably. There is currently no set guidance and practice varies widely.

We know that both MRE and IUS are accurate for diagnosing and staging Crohn's disease, although patients generally prefer IUS if it is of comparable accuracy. We however do not understand if MRE and IUS can be used interchangeably for treatment response assessment. Such uncertainty can impede patients and clinical teams in making informed management decisions.

Currently, IUS provision is sparse with no formal training programme, leading to a heavy bias for MRE. This approach is more costly, less tolerable to patients, adds pressure to already overloaded MRI workflows, and less environmentally sustainable. The current study may support the drive for comprehensive provision of both MRE and IUS services and training.

Currently, the literature is very limited and largely comprised of small single centre studies (most less than 50 patients), using inconsistent imaging definitions of treatment response, highly varied reference standards and differing follow up intervals. There are several MRE and IUS scores for objective disease assessment which remain to be fully validated for response and their respective agreement is not fully understood.

Ultimately, a major outstanding research question is not if investigators should use MRE or IUS, but rather, when investigators should use them in the management of individual patients. Accordingly, the European Society of Gastrointestinal and Abdominal Radiology (ESGAR) and the James Lind Alliance/Crohn's & Colitis UK identify the current study topic as a key research priority.

The proposed prospective study will be the largest to date to investigate the accuracy of MRE and IUS for classification of treatment response against an expert consensus panel, their agreement in response assessment.

Inclusion Criteria:

• Small bowel or colonic Crohn's disease
• 16 years or older
• Baseline/pre-treatment MR Enterography or Intestinal Ultrasound already performed as part of standard clinical care
• Follow up MR Enterography or Intestinal Ultrasound requested to assess treatment response within 15 months of baseline/ pre-treatment imaging

Exclusion Criteria:

• Interval resection of diseased segment(s)
• Inability to achieve informed consent
• Pregnancy
• Any contraindication/ intolerance to MRI

Patients will be male and female from 16 years and above who have small bowel or colonic Crohn's disease.